Method and apparatus for auto-tuning of a radio fm-receiver

ABSTRACT

In a method of auto-tuning a radio FM-receiver the receiver frequency band is scanned until a FM signal is received meeting criteria for identifying the signal as being of a predetermined quality, particularly coming from a valid FM station. At least during tuning it is permanently established whether or nor the FM signal meets the criteria, whereafter the results thereof are read a predetermined number of times, and the FM signal is only stored if at least most of these times the criteria are met. Particularly, the results are read 10 times and the FM signal is only stored if at least 8 times thereof the criteria are met.

This patent document claims benefit to International Application No.PCT/IB03/02427, filed on Jun. 4, 2003, which is fully incorporatedherein by reference.

The invention relates to a method of auto-tuning a radio FM-receiver byscanning the receiver frequency band until a FM signal is receivedmeeting critera for identifying the signal as being of a predeterminedquality, particularly coming from a valid FM station.

One of the essential features in a broadcast receiver is the auto-tunefunction This enables a user to scan the receiver frequency band foravailable signals, tune them in and store them in presets, all at thepress of a button. Particularly in TV-FM radio combi receivers TV and FMradio frequency signals are converted to baseband video and audiosignals by means of such an auto-tune function. This auto-tune functionis important, for example, for PC (personal computer) TV card makers whowant to provide computer users with the possibility to enjoy broadcastTV and radio while working on their PC's.

The concept of auto-tuning for TV is well established and relativelyerror free, in that a TV signal is a unique type of modulated signal,which amongst other characteristics has a periodic synchronizationpulse. With this it is always possible to discriminate between a TVsignal and a non-TV signal, including short duration, non-periodicnoise.

However, a relatively error free auto-tuning is not well possible in thecase of FM radio. A FM discriminator is not inherently able todifferentiate between a real FM radio signal and an interfering non-FMsignal, which can be frequency stable or random. This results in a casewhere well known auto-tune systems may capture numerous signals andstore them, but perhaps only a small part of them are intended signals.Tuning to the rest of the presets will give just irritating noise.

The first problem lies in the sensitivity of the receiver system, whichis able to detect signals that are close to the noise floor. This meansthat the chance of mistakenly storing a random noise signal that existsat the moment of auto-tune is relatively high. In addition, it alsomeans that a true FM signal that is very weak might also be captured. Inreality once a FM radio signal falls below certain signal strength,commonly known as the FM threshold, it makes little sense to store achannel since the sound quality is simply too poor to be intelligible.

It is known to apply two conditions that need to be fulfilled whendeciding if a signal is a valid FM station. The level of FM-IF signalmust be sufficiently high. This can done, for example, by setting a bitin a status register, which is available to the user. This criterion isindicated in the following as “intensity”. When this condition is met,the AFC (automatic frequency control) status must be checked; if thereceived signal is within the IF bandwidth, the second condition is metand the signal is then stored as a valid station. For the 10.7 MHz IFband, the received signals must be within ±75 kHz of the 10.7 MHz IF.This second criterion is indicated in the following as “AFC window” Aconcept of auto-tuning a radio FM-receiver by scanning the receiverfrequency band until an FM signal is received meeting the above criteriafor identifying the signal of a predetermined I quality, is alreadyknown, for example from the European patent application EP-A-0 430 469.When a received FM signal meets both criteria, this is indicated in thefollowing as I “a positive check result”.

However, in case that a sufficiently strong random interference reachesthe demodulator of the radio FM-receiver, it too may be storedmistakenly as it can momentarily appear within the AFC window.

The purpose of the invention is to prevent capture of such frequencyrandom interferences.

Therefore, according to the invention, the method as described in theopening paragraph is characterized in that at least during tuning it ispermanently established whether or nor the FM signal meets the criteria,whereafter the results thereof are read a predetermined number of times,and the FM signal is only stored if at least most of the times thecriteria are met. It has been found that good results are obtained whenthese results are read 10 times and the FM signal is only stored if atleast 8 times thereof the criteria are met.

The invention further relates to an auto tuning device with means toregister whether or not a FM signal, received in a radio FM receiver,meets criteria for identifying the signal as being of a predeterminedquality, particularly coming from a valid FM station, and counting meansto register in an interval of a predetermined number of times that isregistered whether or not the FM signal meets the criteria, the numberof times the FM signal meets the criteria. The invention also relates toa radio FM-receiver for realizing the above method, comprising the aboveauto-tuning device.

The invention further relates to an algorithm for processing FM signalsin an auto tuning device according to the above, to a computer programcapable of running on signal processing means in a radio FM receiver orcooperating with a radio FM receiver comprising the above auto tuningdevice, and to an information carrier, carrying instructions to beexecuted by signal processing means, the instructions being such as toenable said processing means to perform the above method.

The invention will further be clarified by a specific embodiment andwith reference to the drawing, in which:

FIG. 1 shows a flow diagram of the method according to the invention.

The scanning of the FM band, this is in most of the world the frequencyrange from 87.5 to 108.5 MHz, is performed by means of a localoscillator (LO) signal which is swept in a range from 98.2 to 119.2 MHzin steps of 50 kHz. In the flow-diagram this is indicated by block 1. Atthe beginning of each LO sweep a counter for counting positive checkresults is reset. As soon as a FM signal is received, it is checkedwhether or not this signal meets the signal quality criteria “intensity”and “AFC window”. This is done in 10 iteration steps. When the counteris reset, the number of iterations is reset too. The iteration procedureis started by 1 (block 3), while each time the received FM signal ischecked, the number of iterations is raised by 1. The number ofiterations in this example is limited to 10. As long as the number ofiterations has not reached the value of 10, indicated in block 4, it ischecked whether or not the FM signal meets the criteria “intensity”(block 5) and “AFC window” (block 6). When the FM signal meets bothcriteria, the counter for positive check results is raised by 1 (block7) and, after a short delay, a following check of the FM signal isperformed (block 4). When, after 10 iteration steps (block 4), in thisexample 8 times a positive check has been registered (block 8), thefound FM signal is coming from a real station and the correspondingradio frequency thereof is stored (block 9). When 10 iteration steps areperformed and there are no 8 positive results (blocks 4 and 8), the LOfrequency value is raised by two steps, this is by 100 kHz (block 10).When the end of the LO frequency sweep is not yet reached (block 11),the counter is reset again and a search to a valid FM station is startedon a shifted frequency. When the end of the LO frequency sweep isreached, the procedure ends and can be started again. When the firstcriterion “intensity” is not met (block 5), the local oscillator signalis raised, whereafter the procedure is repeated or ended as described.When the first criterion “intensity” is met, but the second criterion“AFC window” not, the number of iteration steps is raised but, ofcourse, not the number of positive results. Nevertheless, thereafter, apositive check result can be obtained, as it is still possible that 8positive check results in 10 iteration steps are obtained.

This procedure is performed in an auto tuning device, being part of aradio FM receiver, with means to register whether or not a FM signal,received in a radio FM receiver, meets the criteria “intensity” and “AFCwindow”. Whether or not the FM signal meets the critera can permanently,or at least during tuning, be registered in the auto tuning device, forexample in a status register therein. This status register will be read10 times during each checking procedure. Therefore, the tuning devicewill comprise counting means to register in an interval of apredetermined number of times (10 times) that is registered whether ornot the FM signal meets the criteria, the number of times the FM signalmeets the criteria.

The above checking procedure will be performed by means of an algorithmin the form of a computer program capable of running on signalprocessing means in the radio FM receiver or cooperating with the radioFM receiver comprising the auto tuning device.

The invention is not restricted to the described embodiment.Modifications are possible. Particularly the number of iteration stepsand the number of positive checking results within the number ofiteration steps may be chosen differently from 10 and 8, respectively.Further, it can be noticed that the scanning procedure as describedabove can be applied for of a FM band in the frequency range from about75 MHz to 91 Mhz, as is the case in Japan.

1. Method of auto-tuning a radio FM-receiver having a receiver frequencyband, the method comprising: scanning the receiver frequency band untila FM signal is received that has a signal strength greater than a FMthreshold and that is in an automatic frequency control (AFC) windowassociated with a valid FM station; immediately after receiving said FMsignal, testing whether the FM signal meets criteria, including checkingwhether the signal strength of the FM signal is greater than the FMthreshold, when the signal strength of the FM signal is greater than theFM threshold, checking whether the FM signal is in the AFC window, andincrementing a count when the FM signal is in the AFC window; repeatingthe testing step a predetermined number of times; and storinginformation denoting a frequency of the FM signal when the countindicates that both of the criteria are met a majority of thepredetermined number of times.
 2. Method as claimed in claim 1, whereinthe predetermined number of times is 10 and the information denoting thefrequency of the FM signal is stored when the count is at least
 8. 3.The method of claim 1, wherein scanning the receiver frequency bandincludes scanning a frequency range from 87.5 to 108.5 MHz.
 4. Themethod of claim 1, wherein scanning the receiver frequency band includessweeping a local oscillator (LO) signal in a range from 98.2 to 119.2MHz in steps of 50 kHz.
 5. The method of claim 4, further comprisingperforming the testing step the predetermined number of times for eachLO sweep and, at the beginning of each LO sweep, resetting the count tozero.
 6. The method of claim 5, wherein the predetermined number oftimes is 10 and, for each LO sweep, storing information denoting thefrequency of the FM signal when the count is at least 8.